A secondary battery has been known to deteriorate as it is repeatedly charged and discharge for use. Therefore, it is important to diagnose deterioration on-board in parallel to use of the secondary battery.
For example, Japanese Patent Laying-Open No. 6-89745 (PTL 1) describes a secondary battery system calculating by estimation the number of remaining charge and discharge cycles of a secondary battery. According to PTL 1, in a battery group constituted of a plurality of secondary batteries connected in series, a remaining cycle life is estimated based on the number of cycles until a set value is reached, by extrapolating a discharge capacity for each cycle or a rate of change in voltage at the time when discharging ends.
In addition, Japanese Patent Laying-Open No. 2006-197765 (PTL 2) describes pricing of a moving vehicle incorporating an electric drive device represented by a secondary battery. According to PTL 2, an ECU (Electronic Control Unit) calculates and stores a deterioration estimation parameter on-board based on data of history of use of a main battery. Then, a deterioration parameter is output from the ECU through a connector and a transmitter to a deterioration estimation device outside a vehicle. The deterioration estimation device estimates a state of deterioration and a remaining life of a battery based on the read deterioration estimation parameter and assesses value of the secondary battery based on this result of estimation.
Japanese Patent Laying-Open No. 2007-195312 (PTL 3) describes a secondary battery life estimation device for estimating a remaining life, which is suitable for a secondary battery mounted on a vehicle. According to PTL 3, a correlation function is determined so as to have a high correlation value with stored full charge capacities or internal resistances of the secondary battery. The correlation function is defined as a linear function having a square root of a total travel distance of a vehicle as a variable and determined by using a least squares method, or the like. Then, a point where the determined correlation function intersects with a life determination line is determined as the life and a travel distance until that life is estimated as a remaining life.
Furthermore, Japanese Patent Laying-Open No. 2008-241246 (PTL 4) describes a technique for diagnosing deterioration of a secondary battery on-line. According to PTL 4, with regard to a parameter in, a battery model equation, a characteristic map in connection with change in new-state parameter value with change in battery state is stored. During use of the secondary battery, a parameter is identified based on the battery model equation. Then, a degree of deterioration of the secondary battery is diagnosed based on a ratio (a rate of change) between the identified parameter value and the new-state parameter value which corresponds to a current state of the battery.